In a power supply system which uses a fuel cell, in order to keep pace with load fluctuations that exceed electric generation capacity of the fuel cell, it is common for electrical power to be supplied by providing a voltage converter which steps up or steps down an output of a secondary battery and by connecting the voltage converter to output terminals of the fuel cell. In such a system, the voltage converter is a converter having a function of direct current voltage conversion and is also referred to as a DC/DC converter, and, for example, a voltage converter composed of switching elements and reactors is used. Then, in view of reducing the rating capacity of the switching element, it is common for a plurality of converters to be connected in parallel.
For example, Japanese Patent Application Laid-Open No. 2006-33934 discloses that, in order to keep pace with rapid changes in load volume that exceed the charging capacity of a fuel cell, a voltage converter that operates in a plurality of phases is connected between the fuel cell and a battery, and a change in the number of phases and a change in duty ratio of the voltage converter are performed by estimating a change in load volume. It is disclosed therein that, generally, in a voltage converter provided with a plurality of phases, loss electric power which is lost when the converter fluctuates according to a value of electric power passing through the voltage converter, which corresponds to input and output conversion energy volume and operational work volume; when the electric power passing through the voltage converter is large, loss of three phase operation having multiple phases than single phase is smaller than that of single phase operation, and when the electric power passing through the voltage converter is small, the loss of the single phase operation is smaller than that of the three phase operation. It is further described the reason as follows. The loss in a three phase bridge type converter includes a reactor copper loss by a reactor coil, a module loss by operation of a switching element, and a reactor iron loss by a reactor magnetic material. And the reactor copper loss and the module loss increase with an increase of the electric power passing through the voltage converter such that the reactor copper loss and the module loss in the single phase operation are larger than that in the three phase operation, while the reactor iron loss does not significantly depend on the electric power passing through the voltage converter, such that the reactor iron loss in the three phase operation is larger than that in the single phase operation. It is further described that the single phase operation is performed in a region where the electric power passing through the voltage converter is small; the three phase operation is performed in a region wherein the electric power passing through the voltage converter is large. And voltage, current, and electric power are temporarily fluctuated in proportional-integral-derivative (PID) control because an effective value of an alternating current for the voltage conversion fluctuates when operation is switched from a three phase operation to a single phase operation; and that therefore a duty ratio is raised temporarily to compensate for a fluctuated shortfall in electric power.
Furthermore, Japanese Patent Application Laid-Open No. 2003-235252 discloses a method of maximizing conversion efficiency In a case wherein a plurality of DC/DC converters is provided between an inverter and a battery. It is described that, in such a case, a master slave DC/DC converter in which one of the plurality of DC/DC converters is set as a master DC/DC converter is provided. And input electric power or output electric power of the master DC/DC converter is set as reference electric power, the number of the DC/DC converters including the master DC/DC converter to be operated is specified. And then conversion efficiencies are calculated when an output voltage of the master slave DC/DC converter is increased and decreased within a range not exceeding the maximum allowable charging voltage and the maximum allowable charging current. And the output voltage is adjusted so as to substantially coincide with the maximum conversion efficiency. In addition, it is descried that the conversion efficiency of the DC/DC converter includes a primary switching loss and a loss depending on a forward voltage drop of a secondary rectifier diode, a primary loss increases at a time of high input electric power, and that at low input electric power the primary loss decreases and a secondary loss becomes dominant.
Japanese Patent Application Laid-Open No. 2003-111384 discloses a method in which prevents that frequency of use of a specified DC/DC converter increases in the case where voltage of electric power of a main power supply is converted by a plurality of DC/DC converters connected in parallel and is supplied to an auxiliary battery. It is described that, in such a configuration, the starting order of each DC/DC converter between the plurality of DC/DC converters is changed in accordance with a predetermined specified order, and that the predetermined specified order sets according to contents that voltage-current characteristics of the respective DC/DC converters are measured.
As described above, in a configuration which is used by connecting a plurality of converters in parallel, control which changes the number of converter phases to be driven in response to the electric power passing through the voltage converter is performed. Furthermore, in order to perform voltage conversion so as to be a desired step-up or step-down, execution is made by controlling a duty ratio of switching pulse. For the control, feedback control or the like, which provides feedback of an actual measurement value of an actually operating duty ratio with respect to a duty ratio command value and suppresses its deviation, is commonly employed.
It should be noted that, when the number of converter phases for use in voltage conversion is changed, the state of its feedback loop is changed, and there are cases where optimum feedback control is not obtained.
An advantage of the present invention is to provide a converter control device capable of performing feedback control suitable for the number of drive phases when voltage conversion is performed by changing the number of drive phases of a converter in response to a load.